To get in line with the global trend of environmental protection, and eco-friendly regulations, electronic product manufacturers nowadays are developing, and manufacturing halogen-free electronic products. Advanced countries, and electronic manufacturing giants set forth schedules of launching mass production of halogen-free electronic products. As a result of the promulgation of the Restriction of Hazardous Substances (RoHS) by the European Union, hazardous substances, such as lead, cadmium, mercury, hexavalent chromium, poly-brominated biphenyl (PBB), and poly-brominated diphenyl ether (PBDE), are strictly prohibited from being used in manufacturing electronic products or their parts, and components. A printed circuit board (PCB) is an indispensable, and fundamental basis of the semiconductor industry, and electronic industry; hence, printed circuit boards bore the brunt of international halogen-free regulations when international organizations set forth strict requirements of the halogen content of printed circuit boards. For example, the International Electrotechnical Commission (IEC) 61249-2-21 requires that bromide content, and chloride content shall be less than 900 ppm, and the total halogen content shall be less than 1500 ppm. The Japan Electronics Packaging, and Circuits Association (JPCA) requires that both bromide content, and chloride content shall be less than 900 ppm. To enforce its green policies, Greenpeace calls on manufacturers worldwide to get rid of polyvinyl chloride (PVC), and brominated flame retardants (BFRs) from their electronic products in order to conform with the lead-free, and halogen-free requirements of green electronics. Hence, the industrial sector nowadays is interested in rendering related materials halogen-free, and sees this technique as one of its key research topics.
Electronic products nowadays have the trend toward compactness, and high-frequency transmission; hence, circuit boards nowadays typically feature a high-density layout, and increasingly strict material requirements. To mount high-frequency electronic components on a circuit board, it is necessary that the substrate of the circuit board is made of a material of a low dielectric constant Dk, and dielectric dissipation factor Df in order to maintain the transmission speed, and the integrity of a signal transmitted. To allow the electronic components to operate well at a high temperature, and a high-humidity environment, it is necessary for the circuit board to be heat resistant, fire resistant, and of low hygroscopicity. Epoxy resin is adhesive, heat resistant, and malleable, and thus is widely applicable to encapsulants, and copper clad laminates (CCL) of electronic components, and machinery. An important factor in the enhancement of heat resistance and flame retardation and assurance of a low dielectric dissipation factor, low hygroscopicity, high cross-linking density, high glass transition temperature, high connectivity, appropriate thermal expansion of circuit boards lies in the selection of an epoxy resin, a curing agent, and a reinforcement material.
Due to rapid development of communication and broadband technology, wide use of cloud computing, and application of high-speed transmission, conventional materials (for example, FR-4) for manufacturing printed circuit boards no longer meet the demand for advanced application, especially the requirements for high-frequency printed circuit boards. Basically, to attain high-frequency printed circuit board high frequency and high-speed electronic transmission characteristics, and yet prevent data loss or interference while transmission is underway, the substrate material used has to conform to the process technology and meet market and application needs in terms of electrical properties, heat resistance, hygroscopicity, mechanical properties, dimension stability, and chemical tolerance. As regards electrical properties, considerations should be given to the dielectric constant Dk and dielectric dissipation factor Df of the material. In general, the signal transmission speed of a copperclad laminate is inversely proportional to the square root of the dielectric constant Dk of the material from which the copperclad laminate is made, and thus the minimization of the dielectric constant Dk of the substrate material is usually advantageously important. The lower the dielectric dissipation factor Df is, the lesser the signal transmission attenuation is; hence, a material of a low dielectric dissipation factor Df provides satisfactory transmission quality.
A conventional insulation film of a printed circuit board essentially consists of a polymeric protective film and an adhesive film. To render the printed circuit board commercially available, it is necessary to adhere a release paper to the adhesive film; hence, the adhesive film is also known as back film. Applications of the back film mainly include protective film of flexible printed circuit boards, insulating protective film of electronic components, and back insulation film of leadframes. Since the aforesaid applications involve a high-temperature process of printed circuit boards, the back film usually includes a polymeric protective film made from polyimide that demonstrates tolerance to high temperature. Furthermore, epoxy resin has excellent electrical characteristics, mechanical strength, and chemical tolerance, and manifests high adhesive capacity toward copper foil and polyimide film, and therefore is often used as an adhesive material for making polyimide-based back film. Therefore, the back film of printed circuit boards is essentially a polyimide-based protective film and has an adhesive made of epoxy resin. Printed circuit boards in operation generate heat, whereas the temperature and humidity in workplace affect the performance and stability of the adhesive; hence, the adhesive not only has to tolerate high temperature and a chemical solvent, but also has to manifest mechanical properties of flexibility and high strength, otherwise heat resistance and stability of the adhesive will compromise the performance of the printed circuit boards. Therefore, resin composition research and development (R&D) has a trend toward achieving high glass transition temperature and low dielectric properties.
Accordingly, it is imperative for printed circuit board material suppliers to develop a material of high heat resistance, low dielectric constant Dk, and low dielectric dissipation factor Df, and being halogen-free, and applies it to the manufacturing of printed circuit boards.